This application requests four years of funding to establish a Conte Center to Develop Collaborative Neuroscience Research. Through the Center, we intend to test a comprehensive, novel hypothesis about the function of sleep - the synaptic homeostasis hypothesis. The hypothesis states that plastic processes during wakefulness result in a net increase in synaptic strength in many brain circuits, leading to increased metabolic consumption. Strengthened brain circuits then lead to larger slow waves during subsequent sleep. In turn, sleep slow waves renormalize synaptic strength to a baseline level that is energetically sustainable and beneficial for memory and performance. Sleep is therefore the price we pay for plasticity, and its function is the homeostatic regulation of the total synaptic weight impinging on neurons. The hypothesis accounts for many facts about sleep and its regulation and makes intriguing predictions that are relevant for both basic and clinical neuroscience. We propose to test such predictions through four tightly linked and complementary projects, to be carried out jointly at the University of Wisconsin and at Washington University. Project I (PI Cirelli) employs a combined molecular / electrophysiological approach in an animal model to establish a relationship between synaptic potentiation during waking, an increase in sleep slow waves, and the resulting synaptic renormalization;Project II (PI Tononi) employs behavioral / high-density (hd)-EEG paradigms in healthy human subjects to determine whether learning leaves a local EEC trace in both wakefulness and sleep, and to determine whether sleep slow waves are necessary to renormalize this trace;Project III (PI Raichle) employs the same behavioral paradigms in conjunction with PET and fMRI to investigate whether learning leaves a local metabolic trace that is renormalized by sleep;and Project IV (PI Benca) employs the same behavioral / hd-EEG paradigms in patients with major depression to evaluate a predicted relationship between preserved slow wave homeostasis and therapeutic response to sleep deprivation. If the hypothesis survives these combined tests, it will provide a scientific explanation of why we need to sleep that ranges all the way from molecular and cellular function to systems neurophysiology and neuroimaging. Given the central role of sleep in the life of every organism, at every age, we expect that the results of our program will have major implications for many aspects of human health and disease.